Failure Investigation of Welded Moulded Plastic Components

Summary

The weld quality of plastics is very much dependent on the material and the joint design for the specific welding process, as well as the welding equipment and the welding procedure. Linear vibration welding, as with most plastic welding techniques, produces flash. TWI has been working closely with a number of automakers and Tier-1 suppliers to major automotive companies. As the result of TWI’s close relationship with the automakers, we have come to see many failures associated with poor welding of moulded components. In this case study we used the general investigatory tools such as visual inspection, pressure/burst test and microtoming. However, these were found to be inadequate and in addition, a non-destructive “X-ray radiography and computed tomography” was utilised with great success.

Background

Global trends in the automotive industry are pushing automakers to reinvent their products and manufacturing processes. The industry is uniting to identify and develop new materials and processes to drive powertrain efficiency. Lightweighting in parts requires replacing conventional metals with lighter polymers and hybrid materials. Replacement of engine components, such as the engine manifold, requires advanced engineering thermoplastic materials, which need to be moulded into a complex 3D structure. A suitable joining process needs to be selected to join such parts together. Weld quality of plastic components is very much dependent on both the material and the joint design for the specific welding process and verification of weld quality has been a challenging task. Most OEMs and Tier-1 suppliers have their own internal welding guidelines. However, an increasing number of premature in-service failures have been reported by the automakers.

At TWI we have utilised common inspection techniques as well as introducing volumetric non-destructive testing of the welded components very effectively.

Objectives

Assessment of weld integrity of a moulded plastic component by destructive and non-destructive examination.

Programme of Work

A vapour separator, made of Hostaform® C13031 (POM), see Figure 1, was welded using linear vibration welding (LVW). This is one of the most widely used welding techniques for thermoplastics for under the bonnet applications.

Verification of weld quality was done by visual inspection and microtoming and, although there were tell-tale signs of poor weld (Figures 2a, 2b, 2c), computed tomography (CT) scans provided the most revealing flaws.

The component was reported to have passed the general QA tests and during the burst test surpassed the bursting pressure requirement of 3 bars and the safety design factor of 5 bars, but still some components failed within the warranty period whilst in service.

Design rules for welding dictate critical dimensions, which should be adhered to if a good weld is to be expected. In most investigations there is no access to the welded locations of the component, therefore their quality cannot directly be compared to the design rules.

Non-destructive qualitative assessment of the welded joint was carried out using an X-Tek HMXCT 225 X-ray Micro-focus machine. Computed Tomography (CT) images and videos were obtained and scrutinised. The information obtained was indicative of poor welding procedures, Figures 3-5.

TWI also helped to set up and develop a new British Standard, BS 89100; “Joining of thermoplastic moulded components.” This is a specification of variables for thermal joining processes, developed to provide companies and organisations with a set of industry approved procedures for the implementation and control of thermal joining processes by specifying the minimum parameters required to produce components to a consistent level of quality.